Annual report pursuant to Section 13 and 15(d)

Redeemable Convertible Preferred Stock - Schedule of Changes in the Preferred Stock (Detail)

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Redeemable Convertible Preferred Stock - Schedule of Changes in the Preferred Stock (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Temporary Equity [Line Items]    
Beginning balance $ 25,871 $ 24,299
Deemed dividends on preferred stock 958 1,572
Conversion Of Preferred Stock In To Common Stock Upon Merger (26,829)  
Ending balance 0 25,871
Series A Preferred    
Temporary Equity [Line Items]    
Beginning balance 3,889 3,689
Deemed dividends on preferred stock 119 200
Conversion Of Preferred Stock In To Common Stock Upon Merger (4,008)  
Ending balance 0 3,889
Series B Preferred    
Temporary Equity [Line Items]    
Beginning balance 5,480 5,174
Deemed dividends on preferred stock 178 306
Conversion Of Preferred Stock In To Common Stock Upon Merger (5,658)  
Ending balance 0 5,480
Series C Preferred    
Temporary Equity [Line Items]    
Beginning balance 16,502 15,436
Deemed dividends on preferred stock 661 1,066
Conversion Of Preferred Stock In To Common Stock Upon Merger (17,163)  
Ending balance $ 0 $ 16,502